This invention relates to a microprocessor-based engine control system and in particular to the circuit that generates a crystal-controlled electrical oscillation from which the microprocessor's clock signal is developed.
Microprocessor-based control systems can be advantageously employed to control certain functions of an automotive vehicle internal combustion engine. Fuel control, ignition control, and throttle control are examples. The improvement of system reliability is a constant objective of conscientious automotive electronic engineers. The present invention has arisen in consequence of the inventor's efforts toward this goal.
Conventional automotive electronic microprocessor technology is configured to utilize a crystal-controlled oscillator to develop the microprocessor's clock signal. The crystal is disposed in external circuitry external to the microprocessor itself, but electrically connected in circuit relation with certain internal circuitry of the microprocessor. These combined circuitries cooperatively form the crystal-controlled oscillator. In order to start the oscillator when the microprocessor is powered-up, a starting resistor of appropriate ohmage and wattage is electrically connected in the external circuitry that contains the crystal.
In seeking improved system reliability, the inventor has recognized that the occurrence of an open-circuit fault in the starting resistor will prevent the oscillator circuit from being started when the microprocessor is powered-up. The consequence is that the microprocessor clock signal cannot be generated and the microprocessor therefore will not operate. In other words the control system will not function.
The inventor proposes to obtain improved reliability against the open-circuit failure of the oscillator starting resistor by what in hindsight is a surprisingly simple means. Rather than embodying the oscillator starting resistor as a single discrete device, it is made to comprise two discrete devices, which in the disclosed preferred embodiment, are two parallel resistors of substantially identical resistance. The relationship is such that 1) the equivalent resistance of the parallel combination of the two discrete resistors is a value that is effective to initiate electrical oscillation of the crystal-controlled oscillator, and 2) the resistance of each single one of the two discrete resistors is effective to also initiate electrical oscillation of the crystal-controlled oscillator so that an open-circuit fault in either one of the two will not prevent the oscillator circuit from being started into oscillation. In other words, open circuit faults must be simultaneously present in both discrete resistors to constitute a disabling fault in the start-up resistance. Accordingly, the reader can readily perceive that improved reliability is attained.
A detailed disclosure will be given in the ensuing description of a presently preferred embodiment constructed in accordance with the best mode presently contemplated for carrying out the invention. A drawing accompanies the disclosure.